1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to an anode for a lithium secondary battery in which adhesive forces among electrode materials and between the electrode materials and a current collector are strengthened to improve the characteristics of the anode and to improve battery performance, and a lithium secondary battery including the anode.
2. Description of the Related Art
Secondary batteries have been used as power sources for portable electrical devices, such as mobile phones, PDAs, laptop computers, digital cameras, camcorders and MP3s, electric-powered cars; and the like. Therefore, the demand for the secondary batteries, which can be repeatedly charged and discharged, has soared. In particular, batteries with high performance are in high demand due to the fact that the portability of these portable electrical devices depends on secondary batteries. Among these secondary batteries, rechargeable lithium secondary batteries have 3 times the energy density per unit weight of Pb storage batteries, Ni—Cd batteries, Ni—H batteries, and Ni—Zn batteries. In addition, rechargeable lithium secondary batteries can be charged rapidly. Due to these advantages, research and development of rechargeable lithium secondary batteries has increased.
A lithium-containing transition-metal oxide, a chalcogen compound, such as MoS2, or the like is under consideration as an active cathode material for lithium secondary batteries. Examples of the lithium-containing transition-metal oxide include LiCoO2, LiNiO2, LiMnO4, and the like. An active anode material for lithium secondary batteries is composed of lithium metal, lithium metal alloy, graphite-based or carbon-based materials, or the like. If an anode is composed of a lithium metal, the volume of a lithium metal changes due to the repeated dissolution and precipitation of lithium during the charge/discharge cycle and needle-shaped lithium dentrite grow locally on the lithium metal. The needle-shaped lithium dentrite functions as dead lithium to decrease the charge/discharge efficiency, and can contact the cathode, which causes a short circuit in lithium secondary batteries.
To solve these problems, a compound that can reversibly intercalate and deintercalate lithium has been suggested as an anode material. Examples of the compound include graphite-based or carbonaceous-based materials, lithium-alloys, metal powders, metal oxides, or metal sulfides. However, if a battery is manufactured using a lithium-alloy anode in a sheet form, the sheet-formed alloy becomes thinner during the charge/discharge cycle, thereby degrading the performance of the current collector. As a result, the characteristics of the charge/discharge cycle of the battery deteriorate.
When the sheet-formed electrode is formed using a metal powder, carbonaceous materials, metal oxides, metal sulfides, or the like, a binder is required in the manufacturing process due to the fact that these materials cannot form the electrode alone. For example, Japanese Laid-Open Patent Publication No. HEI 4-255760 relates to the use of an elastic rubber-based polymer material as a binder in manufacturing an electrode using carbonaceous materials.
In a common method of manufacturing an anode for a lithium secondary battery, an organic solvent containing N-methyl-2-pyrrolidon (NMP) is included in the binder. However, NMP is harmful to humans, which makes the manufacturing process complex. In addition, the organic solvent causes pollution when it is discharged. To solve these problems, Japanese Laid-Open Patent Publication No. HEI 5-74461 relates to a method of manufacturing an aqueous active anode material slurry in which water is used as a solvent, and a synthetic rubber-based latex-type binder and a cellulose-based thickener are used.
However, the use of only the synthetic rubber-based latex-type binder and the cellulose-based thickener in the manufacturing process of an anode does not provide sufficient adhesive forces among electrode materials and between the electrode materials and a current collector. As a result, in a rolling process using a roll press subsequent to a coating process in which the aqueous anode slurry is coated on a copper current collector, the electrode materials are separated from the anode in order to adhere to the roll press. The electrode materials adhered to the roll press scratch the anode in a subsequent rolling process, which causes defects in the battery. In addition, in an assembly process, in which lithium secondary batteries are assembled by rolling the electrode, subsequent to the rolling process, the electrode materials are separated from the anode, which occurs in the corners formed by folding the electrode. Furthermore, if a lithium secondary battery contains an anode having weak adhesive forces, an electrical contact between an active anode material and a copper current collector become weak. As a result, the discharge capacity decreases during discharges at high rates.
Therefore, a great amount of research has been focused on the solutions for weak adhesive forces of the anode, in which the synthetic rubber-based latex-type binder and the cellulose-based thickener are used. For example, the adhesive forces can be increased by increasing the amount of the synthetic rubber-based, latex-type binder. However, in this case, the energy density of the anode is lowered, and electrical conductivity among carbonaceous materials is decreased, thereby lowering the performance of the battery. Therefore, the amount of the cellulose-based thickener available must be limited. Due to these problems, there were other trials to increase the adhesive forces by transforming the physical properties of the carboxymethyl cellulose-based thickener.
For example, Japanese Laid-Open Patent Publication No. 1999-067213 relates to a binder containing a polymer latex and carboxymethyl cellulose (CMC), in which a degree of etherfication (DE) is in the range of 0.5-1 and an average degree of polymerization is in the range of 300-1800. Also, Japanese Laid-Open Patent Publication No. 2002-237305 relates to a binder containing a butadiene-containing rubber and carboxymethyl cellulose, in which the degree of etherfication exceeds 0.65. These techniques attempt to find an appropriate degree of etherfication of carboxymethyl cellulose to enhance the adhesive forces among electrode materials and between the electrode materials and the current collector. However, in these cases, the adhesive forces do not meet the desired level. Furthermore, the electrode materials continue to be separated from the anode and adhere to the roll press during the rolling process.